-
9. SILICEOUS SEDIMENTS FROM THE NAURU BASIN: DIAGENETIC
ALTERATIONOF BIOGENIC OPAL AND AUTHIGENESIS OF SILICA AND
SILICATES1
Volkher Riech, Bundesanstalt für Geowissenschaften und
Rohstoffe,D 3000 Hannover 51, Federal Republic of Germany
ABSTRACT
This study deals with the mineralogical variability of siliceous
and zeolitic sediments, porcellanites, and cherts atsmall intervals
in the continuously cored sequence of Deep Sea Drilling Project
Site 462. Skeletal opal is preserved downto a maximum burial depth
of 390 meters (middle Eocene). Below this level, the tests are
totally dissolved or replacedand filled by opal-CT, quartz,
clinoptilolite, and calcite. Etching of opaline tests does not
increase continously withdeeper burial. Opal solution accompanied
by a conspicuous formation of authigenic clinoptilolite has a local
maximumin Core 16 (150 m). A causal relationship with the lower
Miocene hiatus at this level is highly probable.
Oligocene to Cenomanian sediments represent an intermediate
stage of silica diagenesis: the opal-CT/quartz ratiosof the
silicified rocks are frequently greater than 1, and quartz filling
pores or replacing foraminifer tests is morewidespread than quartz
which converted from an opal-CT precursor.
As at other sites, there is a marked discontinuity of the
transitions from biogenic opal via opal-CT to quartz with
in-creasing depth of burial. Layers with unaltered opal-A alternate
with porcellanite beds; the intensity of the opal-CT-to-quartz
transformation changes very rapidly from horizon to horizon and
obviously is not correlated with lithologicparameters.
The silica for authigenic clinoptilolite was derived from
biogenic opal and decaying volcanic components.
INTRODUCTION
Site 462 is in the deep Nauru Basin, surrounded byatolls of the
Marshall Islands and volcanic islands of theCaroline chain. The
drill penetrated 560 meters of cal-careous and siliceous oozes,
chalks, limestones, andcherts of Tertiary age, and volcanogenic and
zeoliticsandstone, mudstone, and limestone of Maestrichtian
toCenomanian age. Below this sedimentary section werecovered cores
from a thick igneous complex with in-tercalated hyaloclastic
mudstones of Aptian to Barre-mian age. All sedimentary units
contain abundant trans-ported and redeposited sediments.
Previous studies on chert from the western, central,and northern
Pacific include those by Heath and Mo-berly (1971), Lancelot
(1973), Garrison et al. (1975),Keene (1975, 1976), Kelts (1976) and
Hein et al. (1978).Whereas these authors investigated samples from
sev-eral sites, this study covers only one site. However, Site462
offered the opportunity to study the mineralogicalvariability of
siliceous and zeolitic sediments and chertsat small intervals in
one continuously cored sedimentarysequence.
Instead of a detailed description of silicified sedi-ments, I
shall emphasize in this study (1) the behavior ofbiogenic opal
during increasing burial, (2) the min-eralogy of porcellanites and
cherts, particularly the ratioof opal-CT to quartz and the quartz
occurrences withand without detectable opal-CT precursors, and (3)
thesource materials for the authigenesis of zeolites.
The classification of silicified sediments is based on
asemiquantitative estimate of silica mineralogy by opticaland/or
X-ray-diffraction (XRD) analysis. Hence the
Initial Reports of the Deep Sea Drilling Project, Volume 61.
term "porcellanite" is used strictly in a mineralogicalsense for
rocks containing more opal-CT than quartz.
BIOGENIC OPAL IN PLEISTOCENETO EOCENE SEDIMENTS
X-ray-amorphous matter occurs in the entire sedi-mentary column,
but biogenic opal (opal-A) is pre-served only down to a maximum
burial depth of 390meters in Core 462-41 (middle Eocene). Above
Core 41,the contents of X-ray-amorphous material in smectite-poor
samples are mostly identical with the amounts ofoptically
determined biogenic opal. Below the above-mentioned core, amorphous
constituents cannot beequated with biosiliceous components.
Unfortunately,the X-ray-diffraction patterns of opal-A and
otheramorphous constituents could not be distinguished.Opal-A seems
to be detectable by XRD analysis only ifit exceeds 10% (as
estimated from smear slides). Insmectite-rich samples, XRD analysis
may yield overesti-mated percentages of opal-A, because smectite
con-tributes also to the broad "opal hump" between 3 and6Å, and
other X-ray-amorphous matter is more abun-dant in clayey
sediments.
A relatively time-consuming method to study themineralogy of
opaline tests in detail is the measurementof their refractive
indexes (Hurd and Theyer, 1977;Riech, 1980). With some experience,
in many cases asimple visual inspection of cleaned samples under
themicroscope is sufficient to decide whether the fossils
stillconsist of the original opal-A or were diageneticallyaltered
to opal-CT. Usually, remains of opal-A havemuch smoother outlines
and surfaces than opal-CTskeletons.
Figure 1 shows the distribution, abundance, and min-eralogy of
siliceous microfossils. Whereas opal-A-
523
-
V. RIECH
Pleist
Plio.
. O
ISO
I c•M . 2
co Q.X <
Core
10
15
25
30
35'
40
45
H1
H2
40'
46
100 -
2 0 0 -
300 -
4 0 0 -
6 0 0 -
7 0 0 -
VolcanicMaterial"
Abundance (%) andMineralogy of
_i Sediment
-z-z-
500
RCA
Radiolarians
20 40 60 80 100 51015
A
A
[ACTA
Z, CT
[Z, Ca
fZ,(CT)Z, Ca, CT
z Z, CT" Ct, Z, Ca, Q
Q
CT
Q.
— A
SpongeSpicules
10 20
CT
— CT, Q
Etchingof
OpalineTests
1 2 3
SilicifiedSediments
Opal-CT
Quartz
0 1 oo
Opal-CT .Quartz
Trans-formation
1 2 3
AuthigenicZeolitesand Sup-
posed SourceMaterial
R C A
V(B)VV, BV(B)V
ß(V)
, B
VB.?VV,BV
V(?B)
V
V
V(?B)
Figure 1. Pattern of silica diagenesis at Site 462 with respect
to abundance, mineralogy, and etching of siliceous tests; frequency
of volcanic materialand authigenic zeolites and their supposed
source material; variation of opal-CT-to-quartz ratio in
porcellanites and cherts; and the degree of"maturity" of diagenetic
silica phases (inversion from opal-CT to quartz). Abbreviations:
volcanic material in sediments and authigeniczeolites: R = rare, C
= common, A = abundant; A = opal-A, CT = opal-CT, Q = quartz, Z =
zeolitic casts, Ca = calcitic casts ofradiolarians; B = biogenic
source, V = volcanogenic source (volcanic glass, etc.) of zeolites;
etching of opaline tests: 1 = weak, 3 = strong;opal-CT to quartz
ratio: O = pure quartzose cherts, oo = pure opal-CT porcellanites;
opal-CT — quartz transformation: 1 = small amount ofopal-CT
transformed to quartz, 3 = completed transformation. Lithologic
symbols are explained in the site chapter (this volume).
524
-
SILICEOUS SEDIMENTS
preserved radiolarians and sponge spicules are commonand
abundant down to Core 462-41, the diatoms haveonly one maximum in
Cores 462-3 to 5; they are missingin Cores 462-13 to 17, and occur
in lesser amounts be-tween Cores 462-17 and 33. The disappearance
of dia-toms below the Oligocene/Eocene boundary roughlycoincides
with the earliest precipitation of significantamounts of opal-CT in
porcellanites.
In the studied section of Site 462, the fragmentationof skeletal
opal is generally high (Plate 1, Figs. 1 and 2),but does not show a
clear correlation with the intensityof opal dissolution. This means
that the fragmentationis essentially caused by depositional
conditions (e.g.,reworking), and not by diagenesis. Nevertheless,
com-paction and post-burial dissolution have contributed tothe
extremely high fragmentation of opaline tests in themore deeply
buried, semi-consolidated radiolarian oozes.According to sieve
analyses, the siliceous oozes betweenCores 462-1 and 17 contain 30
to 50 wt. °/o opal 1 (Table 1). Thus, opal-CT is gen-erally more
abundant than quartz. One fifth of thestudied samples are pure
quartzose cherts. Only onechert is associated with clayey-siliceous
host sediments(in Core 462-60, immediately above the basalt
sill),whereas the others have calcareous source rocks. Thecherts
consist mainly of quartz which recrystallizedfrom on opal-CT
precursor. This intermediate phasemay be preserved in relics, or
can be recognized fromquartz-replaced opal-CT spheres (Plate 1,
Fig. 5). Intwo chert samples, no distinct opal-CT—quartz inver-sion
is evident but high contents of early-diageneticquartz are visible
(Table 1, Samples 462-40,CC and462-45-1, 7-10 cm).
Whereas 28 of my samples contain more than 5%diagenetic quartz,
only 8 specimens show a distinct tocomplete opal-CT—quartz
transition. This means thatin the recent diagenetic stage quartz
replacement offossils and cementing of pores predominates,
andquartz derived from opal-CT is less important.
As shown in Table 1 and Figure 1, the intensity of
theopal-CT-to-quartz transformation changes very rapidlyin a
vertical direction; obviously, it is not correlatedwith clay or
CaCO3 contents or other parameters seen inthin-sections.
525
-
V. RIECH
Table 1. Characteristics of silicified sediments at Site 462
according to host-rocklithology, opal-CT-to-quartz ratio, and the
origin of the quartz phase.
Sample(interval in cm)
462-32-2, 64-66462A-H1.CC462-34.CC462-4O.CC
462A-H2-1, 40-43
462A-H2-1, 55-60 (1)
462A-H2-1, 55-60 (2)
462A-H2-1, 79-83462A-H2-1, 131-135
462-42-1, 1-4
462-42-1, 8-10
462-42-1, 24-27
462-42-1, 42-44462-42-1, 51-54
462-43-1, 13-15462-43-1, 39-41462A-3-1.24462-3-1,23-25
462A-3-1, 37-39
462-44-1, 2-4
462-4-1, 21-23
462^4-1, 29-32462-44-1, 44462-44-1, 66-67462A-4-1, 2-5462A-4-1,
32-36462A-4-1,44-48
462A-4-1, 62-64462A-4-1, 72-74462-45-1, 7-10
462-47.CC (1)462-47.CC (2)462A-7-1, 5-7462A-7-1, 39462A-7-1,
127-129
462A-9-1, 90
462A-12-1, 31-35462-59-1, 13-15462A-14-1, 120
Lithology ofSilieified Rock
Radiolarian porecellaniteCalcareous porcellaniteRadiolarian
porcellaniteOpal-CT-bearing quartzose
chertPorcellanite with quartzose
patchesPorcellaneous radiolarian
claystoneCalcareous quartz-bearing
porcellaniteRadiolarian porcellaniteForaminiferal arenite
with
quartzose patchesForaminiferal limestone
with opal-CT- andquartz-bearing layers
Calcarenite, partly replacedand cemented byopal-CT and
quartz
Foraminifer-nannofossillimestone impregnatedby opal-CT
Quartzose chertForaminifer-nannofossil
limestone impregnatedby opal-CT
As aboveCalcareous porcellaniteQuartzose chertRadiolarian
porcellanite
with quartz-replacedsponge spicules
Foraminiferal arenite withopal-CT and quartz
Foraminiferal limestoneimpregnated by opal-CT
Foraminiferal limestonewith opal-CT andquartz
Quartzose chertOpal-CT-bearing limestoneQuartzose
chertRadiolarian porcellaniteQuartzose chertOpal-CT-bearing
quartzose
chertMarly porcellaniteMarly porcellaniteOpal-CT-bearing
quartzose
chertQuartz-bearing porcellaniteQuartzose chertQuartz-rich
porcellanitePorcellaneous limestonePorcellaneous foraminifer-
nannofossil limestoneZeolitic radiolarian clay-
stone with opal-CTAs aboveRadiolarian porcellaniteQuartzose
chert, rich in
radiolarians and goethite(above basalt sill)
Main Lithology of
SedimentCalcareous
+ + + +
4-4-4-4-
+ + + +
4-4-4-4-
+ + + +
4-4-4-4-
4-4-4-4-
+ + + +
+ + + +4-4-4-4-
+ + + +4-4-4-4-4-4-4-4-
+ + + +
+ + + +
+ + + +
+ + + ++ + + +4-4-4-4-
+ 4- 44-4-4
4- 44- 4
4 4-4
4 - 4
4- + 44- +4- +
4-
4-
4-
4-
+ 4- + 4-
Siliceous
4-4-4-4-+
4-4-4- +
4-
+ 4- + +
4-
+ + + +
+
4-4-
4-4-4-4-
4-
+
+
4-4-4-4-
4- +4-4-
+ +
4-
+4-
4-4-4-4-
4-4-4-4-4-4-4-4-4-4-4-
Abundanceof Early
DiageneticQuartz
4-+
4-4-4-
+
4- 4-
4-4-4-
4-4-
4-4-4-
+ 4
4- 4-
4-4-
4- +4-
4- 4-
4- +
4-4- +
4- +
4- +
4-4-4-
7?
4- 4-
4-4-4-
4-4-4-
4- 4-
4- 4-
4-4-4-
4-
4-4-4-
4-4-
4-4-
4- +4-4-
Opal-CTQuartz
0 0
> > 1> > l
< l
> 1
> > l
> l
> > l< l
1
< 1
< l
ü> l
> 1> l
0> l
1
> l
1
0> > l
0> l
0< 1
> l> l< l
> l01
> > l> l
> l0
Stage ofOpal-CT-
QuartzTransformation
?
4- +
7
4-
4-
4-4-4-4-
4-4-4-4-
+
4-4-4-4-
4-4-4-4-
4-4-4-4-4-4-4-
4-
4-
4-4-4-4-4- +
+
4-
4-4-4-4-
Note: Stage of opal-CT—quartz transformation: 4- = small amount
of opal-CT starts to transform into quartz, 4-4-4-4- = allopal-CT
transformed into quartz.
Ordering of the Opal-CT Structure
At Site 462 the tf(lθl) spacing of opal-CT decreasesfrom 4.13 Å
(300 m burial depth) to 4.05 Å (560 m burialdepth). This confirms
the well-known phenomenon ofprogressive ordering of the opal-CT
structure with in-creasing age and temperature (see also Murata and
Na-kata, 1974; Murata and Larson, 1975; von Rad et al.,1977).
Density and PorosityDensity and porosity values are mainly
available for
mixed types of porcellanites and cherts and show a widescatter.
As expected, the values of the few measuredpure opal-CT and
quartzose varieties show distinct dif-ferences:
Wet-BulkDensity(g/crrw)
Porosity
PorcellanitesQuartzose cherts
2.07-2.122.50-2.68
13.2-17.70.9-5.2
Relationship between Carbonate and Silica DiagenesisVan der
Lingen and Packham (1975) investigated the
carbonate sequence of the Ontong-Java plateau andsuggested that
silica diagenesis affects carbonate dia-genesis, because "the
replacement of calcite by silicamust result in calcium carbonate
enrichment outside thechert nodule." Likewise, at Site 462 a mutual
influencecan be recognized. In Cores 462-42 to 47, strong
silici-
526
-
SILICEOUS SEDIMENTS
fication results in increasing CaCO3 cementation of inter-bedded
chalks and limestones. Furthermore, smear slidesshow that below 300
meters burial depth, where the firstporcellanites occur, the ratio
of identifiable nannofossilsto unspecified carbonate particles
decreases, and in themain cherty interval the unspecified carbonate
predomi-nates. Below this level of widespread silica
precipitation,nannofossils are again better preserved.
ZEOLITIC SEDIMENTSX-ray-diffraction analyses of 13 zeolitic
samples from
various lithologies constantly yielded clinoptilolite as theonly
zeolite. Maximum percentages with up to 90%clinoptilolite occur in
zeolitic claystones which wereoriginally rich in radiolarians. This
is evident from poorlyrecognizable zeolitic fillings of dissolved
radiolarianskeletons, whereas in these beds alteration products
ofvolcanic origin are lacking or of minor importance.However, it
has to be admitted that sometimes a differen-tiation between former
volcanic and biosiliceous com-ponents in altered zeolitized
sediments is impossible.
The youngest sediments containing an abundant ad-mixture of
clinoptilolite occur in Core 462-16. As men-tioned earlier, several
horizons with strong dissolutionof biogenic opal occur in this
core. In addition to zeo-litic radiolarian casts, clinoptilolite
usually forms tiny,poorly developed prisms (Plate 1, Fig. 6). The
zeolite-rich beds (up to 60% clinoptilolite) in Core 16
coincidewith the minimum of opal-A-preserved skeletons. Inlevels of
lesser corrosion, there are only trace amountsof zeolites. Because
a few horizons with nearly completedissolution of opal do not
contain any clinoptilolite, thedegree of corrosion is not
necessarily correlated with theformation of zeolite.
Although clinoptilolite in Core 462-16 is associatedwith
(dissolved) siliceous organisms, most of the radio-larian-rich
porcellanites do not show this mutual occur-rence. In general,
zeolites are only sporadically distri-buted in the cherty zone
(Fig. 1); this is also true foropal-CT in the zeolite-rich sequence
below 450 meters.Consequently, the distribution of authigenic
silica isrelatively independent of that of clinoptilolite. This is
incontrast with many other DSDP occurrences (e.g.,Stonecipher,
1976; Hein et al., 1978; Riech, 1980).
Zeolites appear mainly in the volcanogenic sectionbetween the
basalt sill and the cherty zone, and in someintra-basalt mudstones.
The zeolitic sediments representthe alteration products of
volcaniclastic siltstones andsandstones, tuffaceous bioarenites,
vitric tuffs, ashymarlstones, hyaloclastites, radiolarian clays,
and nan-nofossil limestones with admixtures of radiolarians
andvolcanic components. Frequently, small zeolite prismsform the
matrix together with authigenic smectite. Insiltstones or
sandstones, larger clinoptilolite crystals oc-cur (Plate 1, Fig.
7), which grew in open cavities. At cer-tain levels, clayey,
calcitic, or zeolitic radiolarian castsare common in volcanogenic
sediments. Furthermore,there are some bluish-green and brown beds
of zeoliticclaystone, only a few centimeters thick, which are
inter-preted as the products of background pelagic sedimen-tation
and probably were very rich in radiolarians
before zeolitization (e.g., in Cores 462-49, 52, 53, 54,55, 58;
462A-9, 11, and 12).
AUTHIGENIC CLAY MINERALSThe following comments on the
distribution and
origin of clay minerals at Site 462 are based on
X-ray-diffraction analyses of 45 sediment samples (H.
Rösch,Hannover). Smectite is the most abundant clay mineral;it
occurs throughout the sedimentary sequence. Accord-ing to Eslinger
and Savin (1976) and Hein and Scholl(1978), the average percentage
of expandable layers inthe illite/smectite of a sample is a clue to
the origin ofthis mineral. Our analyses confirm that the Tertiary
se-quence of Site 462 contains mostly detrital illite/smec-tite
(l/h: 0.36-0.70), whereas in the Cretaceous sedi-ments with
abundant volcanic parent material authigenicsmectite appears to be
more frequent (l/h: 0.71-0.89).
Palygorskite occurs in larger proportions only in thevolcanic
sequence (e.g., in Cores 462-51, 55, and 462A-10). The genesis of
palygorskite in deep-sea sediments isstill somewhat enigmatic
(Weaver and Beck, 1977; Cou-ture, 1978). In this case, it is
possible that the hydrousmagnesium silicate originated from
degraded mont-morillonitic clays in the presence of
magnesium-andsilica-rich solutions derived from the decay of
volcanicparticles and radiolarians.
CONCLUSIONS
Paleoenvironment and Silica DiagenesisDuring the northwestward
motion of the Pacific
Plate, the area of Site 462 reached the equatorial zone ofhigh
productivity (~5°S) approximately at the begin-ning of the Eocene,
and remained between 5°S and 5°Nuntil the late Miocene (compare
Lancelot and Larson,1975, fig. 1). The crossing is marked by
sediments richin siliceous microfossils. Nevertheless, radiolarians
weredeposited in significant amounts already in Cenomaniantimes at
a paleolatitude of 25 to 30°S, causing the for-mation of the oldest
cherts at Site 462.
In comparison with the DSDP sites in the northwest-ern Pacific
(Leg 32), the Cretaceous at Site 462 is com-paratively poor in
"cherts!' This might be explained bysynchronous volcanism and the
accumulation of abun-dant volcaniclastic material and basalts in
the NauruBasin during Cretaceous times: during the alteration
ofvolcanic particles and the authigenesis of smectite andzeolites,
less free silica is available to form porcellanitesand cherts.
The deposition of abundant calcareous turbidites inthis deep-sea
environment below the CCD did .not fun-damentally influence chert
diagenesis in the Oligoceneto Maestrichtian section. However, in
some layers thecalcareous matrix accelerated conversion of
porcel-lanites to quartzose cherts, and foraminifers and
cal-careous shallow-water debris were preferably replacedby
quartz.
Pre- and Post-Burial Dissolution of OpalThe maximum opal
solution above the chert horizon
in Site 462 occurs in Core 16. A causal relationship be-
527
-
V. RIECH
tween the complete disappearance of opaline skeletonsdue to
pre-burial solution, the lower pliocene hiatus,and the formation of
authigenic clinoptilolite is highlyprobable. This finding appears
to be an exception to thestatistically proved correlation between
clinoptiloliteand moderate to high sedimentation rate in the
hostsediments at many DSDP sites (Stonecipher, 1976).
The possibility of post-burial and deposition- inde-pendent
influences is very unlikely, because of the sud-den increase of
opal solution in this core and the occur-rences of better-preserved
skeletons below this level.Von Stackelberg (1979) detected a
similar relationshipnear a widespread Miocene/Pliocene hiatus in
the equa-torial Pacific southeast of Hawaii with high amounts
ofradiolarian casts and newly formed phillipsite. Riedel(1959)
attributed the poor preservation or absence ofsiliceous
microfossils to low sedimentation rates. In adetailed study of the
dissolution of radiolarians in sedi-ments of the Ontong-Java
plateau, Holdsworth andHarker (1975) found that some periods of
intensifiedcorrosion were probably periods of non-depositionand/or
erosion.
During the dissolution of opaline components in theuppermost
layers of a sedimentary sequence, either thesilica-enriched waters
escape more or less completelyinto the oceanic bottom water, or
some dissolved silicais consumed to form authigenic silicates such
as zeolitesor smectite (Johnson, 1976). No formation of such
min-erals is expected if biogenic opal is dissolved immedi-ately at
the sediment surface or during redeposition.Most probably,
clinoptilolite, phillipsite, or smectiteform only in
sediment-covered layers which allow thepore waters to become
enriched in silica and delay theflux of solutions into the
overlying bottom waters. Thealternation of dissolution levels with
and without clin-optilolite in Core 462-16 can be explained by
differentsediment covers during the solution of opal and forma-tion
of zeolites, but the mechanisms of zeolite for-mation near the
water/sediment interface during pe-riods of low sedimentation or
nondeposition are stillpoorly understood.
A few radiolarian porcellanites are enriched in fishremains, as
are horizons of strong opal dissolution inCore 462-16. This
indicates low sedimentation rates. Agenetic connection between
unconformities pre-burialopal dissolution, and formation of
porcellanites is notsupported by DSDP evidence. The best
prerequisite forsignificant post-burial silicification is a
moderate tohigh sedimentation rate, which favors preservation
ofbiogenic silica, before diagenetic corrosion starts (Riechand von
Rad, 1979a).
The disappearance of biogenic opal below Core 462-41 resembles
the supposed pre-burial dissolution of sili-ceous microfossils in
Core 16. Unfortunately, in sedi-ments of increasing age and/or
deeper burial it becomesmore and more difficult to differentiate
between pre-burial and post-burial corrosion of opal, because
post-burial dissolution masks a possible earlier stage of etch-ing.
In Core 41 and below, diagenetic dissolution proc-esses are
probably more important; this is indicated bythe strong
silicification of these horizons. According to
our experience from the Atlantic Ocean, biogenic opalmay be
preserved in middle Eocene sediments down to aburial depth of 600
to 700 meters, but in many cases theopal-A—opal-CT conversion takes
place at shallowerdepths, so that the fact that opal-A at Site 462
is buriednearly 400 meters fits well in the age-depth diagram
ofsilica phases from the Atlantic Ocean (Riech and vonRad, 1979b,
fig. 11).
Opal-CT/Quartz Ratios of Silicified Sediments
A semiquantitative estimate of the opal-CT/quartzratio is
sufficient to describe the mineralogy of silicifiedrocks. In
addition, the genesis of quartzose componentshas to be optically
analyzed to avoid misinterpretationof the diagenetic history of
"cherts!' It seems importantto differentiate between early
diagenetic quartz and re-crystallized quartz which originated in
situ from an opal-CT precursor. In general, the opal-CT/quartz
ratio iscontinuously reduced to lower values with
proceedingdiagenesis.
During silicification, the opal-CT/quartz ratio is es-sentially
controlled by the original sedimentary compo-nents and the quantity
and size of pores or voids. Opal-CT is usually the first silica
cement. Because the crystal-lites and spheres are small, they fill
only micropores(e.g., within a nannofossil matrix) and marginal
parts oflarger voids. Oozes composed of highly fragmentedopaline
debris, as at Site 462, are converted to porcel-lanites with
extremely high opal-CT/quartz ratios.Quartz crystallizes in the
central parts of fossil pores orother larger cavities; it replaces
foraminifer tests, cal-citic fragments of shallow-water debris, and
spongespicules. Therefore, foraminiferal arenites and radio-larian
oozes with many complete radiolarians (i.e.,many fossil voids)
and/or sponge spicules are rich inquartz, precipitated during early
diagenesis. Opal-CT-cemented nannofossil oozes, clays and sediments
withbroken radiolarian debris do not leave any pore spacefor the
crystallization of chalcedony. In these sedi-ments, quartz occurs
only after it is converted fromopal-CT in a later diagenetic
process.
In the actual diagenetic stage, the opal-CT/quartzratios of the
silicified rocks at Site 462 increase fre-quently to more than 1,
and early diagenetic quartz isstill more widespread than quartz
which converted froman opal-CT precursor. Thus, an intermediate
stage ofsilica diagenesis is indicated in the Oligocene to
Ceno-manian sediments, although in a few horizons the "mat-uration"
of metastable opal-CT to stable quartz is al-ready completed.
According to Heath (1973) and Keene(1975), opal-CT also
predominates in carbonate-richsediments of Legs 16 and 32, whereas
Lancelot (1973)postulated that, in general, calcareous
environmentsfavor precipitation of quartz. Doubtless the main
lithol-ogies influence the opal-CT/quartz ratio, especially dur-ing
late diagenesis. The conversion of opal-CT to quartzis accelerated
in calcareous host sediments, and retardedin clays. This is also
confirmed by our studies of Site 462material. In contrast to the
experimental data ofKastner et al. (1977), the opal-A — opal-CT
trans-formation appears not to be significantly accelerated in
528
-
SILICEOUS SEDIMENTS
the carbonates of the Nauru Basin. However, theauthors also
state that because of different rates of dif-fusion of CO2 out of
the system the acceleration ofopal-A diagenesis in carbonate layers
is not necessarilyobserved.
Influences of Age and Temperatureon Silica Transformations
In addition to facies, sediment age and depth ofburial or
temperature are the most obvious factors in-fluencing the rate of
chertification. In Figure 2, thepresent depth of the youngest
sediment with authigenicsilica is plotted against age for many
Pacific DSDPsites. In general, the sediment age seems to be a
moreimportant diagenetic factor than burial depth. The Mio-cene
porcellanite occurrences in the marginal basins ofthe Pacific (area
encircled by dashed line in Fig. 2) areexceptions, because high
heat flow and/or high amountsof less-resistant diatoms accelerate
the precipitation ofopal-CT (Keene, 1976). Even in these cases, a
minimumage of 10 m.y. is often necessary to initiate
silicification.At least 25 m.y. pass after the deposition of
opal-bear-ing sediments before silicified rocks form in the
centralregions of the Pacific. In addition to age and burialdepth,
we have to take into account highly differentheat-flow values,
varying sediment types, and other un-known parameters controlling
silica diagenesis. There-
100-
30 40Age (m.y.)
70
Figure 2. Present depth of burial of the youngest Pacific
sedimentcontaining authigenic silica (DSDP Legs 5-9, 16-21, 30-33,
35, 61,and 63). Sites at which no sediments with biogenic silica
overlie thesilicified horizons and sites with a major disconformity
wereomitted.
fore, we see no clear correlation between sediment ageand depth
of burial of the young porcellanites (Fig. 2).
In the clay stones of Site 462, no quartzose chert wasexpected,
because it is well known that clayey matrixsignificantly retards
the conversion of opal-CT toquartz. During the formation of the
very dense andtough chert in Core 462-60 immediately above the
basaltsill, the retarding effect of the clay minerals was proba-bly
compensated by the thermal influence of the intru-sion, which
accelerated crystallization of quartz fromthe opal-CT
precursor.
The Problem of Discontinuous Silicificationwith Increasing Depth
of Burial
We have briefly discussed the possible influences ofsediment
age, depth of burial or temperature, and lith-ology on the
transformation rates of the silica phases.Nevertheless, in many
cases the marked discontinuity ofthe transitions from opal-A via
opal-CT to quartz withincreasing depth of burial is a puzzling
problem. At Site462, this is especially obvious at the level of
Cores 462-41and 42, where layers with mineralogically
unalteredopal-A alternate with porcellanite beds, and even withone
quartzose chert. Below this section, we observeagain strong
fluctuations in the degree of the opal-CT—quartz conversion:
calcareous porcellanites alternatewith calcareous cherts (Table 1;
Fig. 1), and there is noevidence that simple lithological factors,
such as the car-bonate or clay content, are responsible for the
dif-ferences.
Apparently, we can apply the concept of the "dia-genetic
potential'' originally developed for ooze-chalk-limestone sequences
(Schlanger and Douglas, 1974),also to the diagenetic evolution of
siliceous deposits.These authors postulate that variation in the
degree ofcementation (plotted as a function of burial depth)
iscaused by initial variations in the diagenetic potential ofthe
original sediment, so that chalks (porcellanites) canform above
oozes (siliceous oozes), and limestones(cherts) above chalks
(porcellanites) in the sedimentarycolumn. According to Schlanger
and Douglas (1974),the diagenetic potential of the sediment is in
large partpredetermined by oceanographic conditions prior toburial.
The same holds true for siliceous plankton,which may produce
several varieties of opal (Goll andBj0rklund, 1972; Hurd and
Theyer, 1977), reacting in adifferent manner under post-burial
conditions.
The Genesis of Zeolites
In contrast to authigenic silica at Site 462, the silicafor
zeolites can be derived from biogenic opal and vol-canic components
(Fig. 1). This means that clinoptiloliteis not a typical product of
decaying volcanic material,and does not clearly indicate a biogenic
origin (comparealso Kastner and Stonecipher, 1978).
Surprisingly, the radiolarian sediments below a burialdepth of
450 meters are rarely transformed into"cherts" (e.g., in Cores
462-59 and 60), but mostly intozeolitic clay stones. This might be
explained by the highratio of volcanic to opaline material. The
diageneticdecay of volcanic material concentrates metal ions in
the
529
-
V. RIECH
pore waters, which favors the crystallization of more-complex
silicates. Possibly the admixture of opalinematerial (pure silica)
in the volcanogenic sequence sup-ported the formation of
silica-rich clinoptilolite andprevented authigenesis of the
less-silica-rich zeolites(e.g., phillipsite) in noticeable
amounts.
ACKNOWLEDGMENTSI am especially grateful to U. von Rad
(Hannover), who reviewed
the manuscript and suggested essential improvements. H.
Rösch(Hannover) carried out X-ray-diffraction analyses and gave
advice onthe interpretation of these data. This paper was also
kindly reviewedby M. Kastner. Financial support from the German
Research Society(DFG grant Ra 191/8) is gratefully appreciated.
REFERENCESCouture, R. A., 1978. Comments on: Miocene of the S.E.
United
States: a model for chemical sedimentation in a peri-marine
en-vironment (Weaver and Beck). Sed. Geol., 21:149-154.
Eslinger, E. V., and Savin, S. M., 1976. Mineralogy and O18/O16
ratiosof fine-grained quartz and clay from Site 323. In Hollister,
C. D.,Craddock, C , et al., Init. Repts. DSDP 35: Washington
(U.S.Govt. Printing Office), 489-496.
Garrison, R. E., Rowland, St. M., Horau, I. J., et al., 1975.
Petrol-ogy of siliceous rocks recovered from marginal seas of the
westernPacific, Leg 31, Deep Sea Drilling Project. In Karig, D. E.,
Ingle,J. C , et al., Init. Repts. DSDP, 31: Washington (U.S.
Govt.Printing Office), 519-529.
Goll, R. M., and Bjtfrklund, K. R., 1972. Radiolaria in
surfacesediments of the North Atlantic Ocean. Micropaleont.,
17:434-454.
Heath, G. R., 1973. Cherts from the eastern Pacific. In van
Andel, Tj.H., Heath, G. R., et al., Init. Repts. DSDP, 16:
Washington (U.S.Govt. Printing Office), 609-613.
Heath, G. R., and Moberly, R., 1971. Cherts from the
westernPacific, Leg 7, Deep Sea Drilling Project. In Winterer, E.
L.,Riedel, W. R., et al., Init. Repts. DSDP, 1, Pt. 2:
Washington(U.S. Govt. Printing Office), 991-1008.
Hein, J. R., and Scholl, D. W., 1978. Diagenesis and
distribution ofLate Cenozoic volcanic sediment in the southern
Bering Sea. Geol.Soc. Am. Bull., 89:197-210.
Hein, J. R., Scholl, D. W., Barron, J. A., et al., 1978.
Diagenesis oflate Cenozoic diatomaceous deposits and formation of
the bottomsimulating reflector in the southern Bering Sea.
Sedimentol.,25:155-181.
Holdsworth, B. K., and Harker, B. M., 1975. Possible indicators
ofdegree of Radiolaria dissolution in calcareous sediments of
theOntong-Java Plateau. In Andrews, J. E., Packham, G., et
al.,Init. Repts. DSDP, 30: Washington (U.S. Govt. Printing
Office),489-495.
Hurd, D. C , and Theyer, F., 1977. Changes in the physical
andchemical properties of biogenic silica from the central
equatorialPacific: Part II. Refractive index density and water
content ofacid-cleaned samples. Am. J. Sci., 277:1168-1202.
Johnson, T. C , 1976. Biogenic opal preservation in pelagic
sedimentsof a small area in the eastern tropical Pacific. Geol.
Soc. Am.Bull., 87:1237-1282.
Kastner, M., Keene, J. B., and Gieskes, J. M., 1977. Diagenesis
ofsiliceous oozes. I. Chemical controls on the rate of opal-A to
opal-CT transformation—an experimental study. Geochim. Cosmo-chim.
Ada, 41:1041-1059.
Kastner, M., and Stonecipher, S. A., 1978. Zeolites in pelagic
sedi-ments of the Atlantic, Pacific, and Indian Ocean. In Sand, L.
B.,
and Mumpton, F. A. (Eds.), Natural Zeolites: New York (Per-gamon
Press), pp. 199-220.
Keene, J. B., 1975. Cherts and porcellanites from the North
Pacific,Deep Sea Drilling Project Leg 32. In Larson, R. L.,
Moberly, R.,et al., Init. Repts. DSDP, 32: Washington (U.S. Govt.
PrintingOffice), 429-507.
, 1976. The distribution, mineralogy and petrography ofbiogenic
and authigenic silica from the Pacific Basin [Ph. D.dissert.].
Scripps Inst. Oceanogr., San Diego.
Kelts, K., 1976. Summary of chert occurrences from Line Island
Sites314, 315, 316, DSDP Leg 33. In Schlanger, S. O., Jackson, E.
D.,et al., Init. Repts. DSDP, 33: Washington (U.S. Govt.
PrintingOffice), 855-863.
Lancelot, Y., 1973. Chert and silica diagenesis in sediments
from thecentral Pacific. In Winterer, E. L., Ewing, J. I., et al.,
Init. Repts.DSDP, 17: Washington (U.S. Govt. Printing Office),
377-405.
Lancelot, Y., and Larson, R., 1975 Sedimentary and tectonic
evolutionof the northwestern Pacific. In Larson, R. L., Moberly,
R., et al.,Init. Repts. DSDP, 32: Washington (U.S. Govt. Printing
Office),925-939.
Murata, K. J., and Larson, R. R., 1975. Diagenesis of
Miocenesiliceous shales, Tembler Range, California. /. Res. U.S.
Geol.Surv., 3:553-566.
Murata, K. J., and Nakata, J. K., 1974. Cristobalitic stage in
thediagenesis of diatomaceous shale. Science, 18:567-568.
Riech, V., 1980. Diagenesis of silica, zeolites and
phyllosilicates atSites 397 and 398. In von Rad, U., Ryan, W. B.
F., et al., Init.Repts. DSDP, 47, Pt. 1: Washington (U.S. Govt.
Printing Office),741-760.
Riech, V., and von Rad, U., 1979a. Eocene porcellanites and
EarlyCretaceous cherts from the western North Atlantic. In
Tucholke,B., Vogt, P. R., et al., Init. Repts. DSDP, 43: Washington
(U.S.Govt. Printing Office), 437-455.
, 1979b. Silica diagenesis in the Atlantic Ocean:
diageneticpotential and transformations. In Talwani, M., Hay, W.,
andRyan, W. B. (Eds.), Deep Drilling Results in the Atlantic
Ocean:Continental Margins and Paleoenvironment: Washington
(Am.Geophys. Union), pp. 98-107.
Riedel, W. R., 1959. Siliceous organic remains in pelagic
sediments.In Ireland, H. A. (Ed.), Silica in sediments: Soc. Econ.
Paleont.Mineral. Spec. Pub., 7:80-91.
Schlanger, S. O., and Douglas, R. G., 1974. Pelagic
ooze-chalk-lime-stone transition and its implications for marine
stratigraphy. InHsü, K. J., and Jenkyns, H. C. (Eds.), Pelagic
Sediments: OnLand and under the Sea: Spec. Pub. Int. Assoc.
Sedimentol.,1:117-148.
Stonecipher, S. A., 1976. Origin, distribution and diagenesis
ofphillipsite and clinoptilolite in deep-sea sediments. Chem.
Geol.,17:307-318.
van der Lingen, G. J., and Packham, G. H., 1975. Relationships
be-tween diagenesis and physical properties of biogenic sediments
ofthe Ontong-Java Plateau (Sites 288 and 289, Deep Sea
DrillingProject). In Andrews, J. E., Packham, G., et al., Init.
Repts.DSDP, 30: Washington (U.S. Govt. Printing Office),
443-481.
von Rad, U., Riech, V., and Rösch, H., 1977. Silica diagenesis
in con-tinental margin sediments off Northwest Africa. In Lancelot,
Y.,Seibold, E., et al., Init. Repts. DSDP, 41: Washington (U.S.
Govt.Printing Office), 879-905.
von Stackelberg, U., 1979. Sedimentation, hiatuses and
developmentof manganese nodules (Valdivia Site VA 13/2, Northern
CentralPacific). In Bischoff, J. L., and Piper, D. Z. (Eds.),
MarineGeology and Oceanography of the Central Pacific
ManganeseNodule Province: New York (Plenum), pp. 555-587.
Weaver, C. E., and Beck, K. C , 1977. Miocene of the S.E.
UnitedStates: a model for chemical sedimentation in a peri-marine
en-vironment. Sediment. Geol., 17:1-234.
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SILICEOUS SEDIMENTS
ar
100µm 100µm
100µπ
100µm
Plate 1. Photomicrographs.
Figure 1. Sample 462-38-1, 28-29 cm (355 m; late Eocene);
photo-micrograph of an undisturbed firm radiolarian ooze; between
fewunfragmented spumellarian tests the matrix is composed of
highlyfragmented opaline particles; the diagnetic alteration is
restrictedto a slight compaction of the ooze and to partial
dissolution of thebiogenic opal; there is a total lack of
authigenic silica cement andno mineralogical alteration of the
original opal-A to other silicaphases.
Figure 2. Sample 462-39-1, 125-127 cm (365 m; early-middle
Eocene);smear slide of the < 32-µm size fraction of a siliceous
ooze; 60 wt.°fo of the bulk sample is composed of opaline particles
in this size;because of common reworking at Site 462, fragmentation
of sili-ceous shells is high.
Figure 3. Sample 462-17-5, 77-79 cm (160 m; early Miocene),
signifi-cant dissolution of delicate opaline components results in
asiliceous assemblage of heavy-shelled fragments of radiolariansand
sponge spicules (see also Fig. 9).
Figure 4. Sample 462A-4-1, 2-5 cm (412 m; early Eocene);
thin-sectionof a radiolarian porcellanite; matrix is
smectite-bearing opal-CTwhich originated from highly fractured
opaline particles similar tothose occurring in the ooze of Figure
1; clear radiolarian cavitiesare filled by tiny lepispheres of
opal-CT and (mostly) chalcedony.
Figure 5. Sample 462-44-1, 29-32 cm (410 m; early Eocene);
quartzosechert replacing foraminifer limestone. Note relict opal-CT
spheres(arrows), now transformed to chalcedony which lines the
formerwalls of foraminifer chambers.
Figure 6. Sample 462-16-4, 135-137 cm (150 cm; early Miocene);
morethan half of this zeolitic clay (smear slide of the < 32-µm
fraction)is composed of tiny, badly developed clinoptilolite
prisms; ex-tremely corroded and dissolved siliceous assemblages in
manysamples of Core 16 can be explained by low sedimentation rates;
inthis case all biogenic opal is dissolved, which resulted in
strongcrystallization of clinoptilolite.
Figure 7. Sample 462A-7-1, 86-89 cm (440 m; middle
Maestrichtian);HCl-residue of altered tuffaceous biocalcarenite
with abundantclinoptilolite; in contrast to the biogenic precursor
material ofzeolites in Figure 6, a mainly volcanic source is highly
suggested inthis sample.
Figure 8. Sample 462-6-1, 99-101 cm (50 m; late Miocene); glass
shardfrom ashy nannofossil ooze with solution pits; associated
radio-larians and sponge spicules are also slightly corroded.
Figure 9. Sample 462-5-4, 80-82 cm (45 m; late Pliocene),
spongespicule from siliceous nannofossil ooze, with very distinct
solutionpits.
531